The present invention relates to an electrical switch, particularly to a high power DC switch, more particularly to a trigger switch used for trigger control of a power tool.
It is publicly known in the art that there is a Marquardt (trademark) 2711.5507 high rating DC switch powered with electricity of 18V DC 25 A. U.S. Pat. No. 6,717,080 discloses a trigger switch assembly used for an electric power tool having an electric motor. U.S. Pat. No. 6,741,051 discloses a trigger controller used for an electric power tool including an electric motor. Also, U.S. Pat. No. 6,104,105 discloses an electrical switch for switching on and off a tool having an electric motor.
A pivot contact lever mechanism is used in all of the above-mentioned switches to switch on and switch off the electric connection. Typically, one end of the pivot contact lever is pivotably attached to a support which is connected to an input terminal for electric current. An output terminal of the electric circuit is attached with a fixed contact. The other end of the pivot contact lever can be pivoted to contact the fixed contact so as to be electrically connected. In these switches, when the pivot contact lever connects the electric circuit, electric current flows through the pivot point between the pivot contact lever and the support, and the pivot contact lever and the fixed contact. However, contact between the pivot contact lever and the support on the pivot point may be not very desirable because it generates a high contact resistance and causes the heating generation and “INT” problem. The INT problem refers to the sparkle that is generated at the contact point due to unstable contact at the contact point under the normal working condition and/or environment. The sparkle, in return, may burn or melt the pivot contact plate, which becomes a more serious problem on a high rating switch.
In addition, the above-mentioned conventional switches have the following deficiency. As a by-pass switch, namely, a second pivot contact lever is provided, when the trigger (a button) is pressed to the innermost position, the second pivot contact lever connects the input and the output. For a high rating switch, the normal operating current is around 25 A. However, the current can be up to 100 A at the very moment when the motor starts or stalls. Thus, even though contact loss is very small for the second pivot contact lever, heat is generated in such a high current situation.
A conventional switch has further deficiency in that a conventional switch is normally provided with a forward/reverse switch mechanism that is able to alternate the pole of the output voltage. Two switch contact plates are provided for the switch mechanism to connect simultaneously two internal outputs to two external outputs, thereby to control the pole of the output. There is only one single convex portion on each internal output that provides a line contact between the switch contact plate and the internal output so as to decrease contact loss. However, such a structure does not effectively minimize heat generation.